* Temperatures statewide plunge by as much as 20 degrees below normal
* Strawberry crop, other produce also faces frost threat
* Snowfall strands motorists along key stretch of highway
By Brandon Lowrey
LOS ANGELES, Jan 11 (Reuters) - An Arctic air mass has sent temperatures plunging across California, threatening the state's lucrative citrus harvest, its winter vegetables and its more cold-sensitive strawberry crop, weather and agricultural experts said on Friday.
Temperatures throughout the state fell by as much as 20 degrees Fahrenheit (11 degrees Celsius) below normal on Thursday and Friday as snowfall and sub-freezing conditions forced a 17-hour closure of a key highway, Interstate 5, through the mountains north of Los Angeles.
Authorities reopened the highway Friday morning after thousands of motorists were left stranded overnight on either side of the winding stretch of road known as the Grapevine or were forced to take circuitous alternate routes.
It remained unclear how much of the state's $2 billion-a-year citrus industry, which accounts for most of the oranges and lemons commercially available to U.S. consumers, might be lost.
In the San Joaquin Valley, the heart of navel and mandarin orange production in central California, low temperatures in the teens threatened to kill citrus crops, which are in danger of perishing whenever the mercury falls below 28 degrees, farmers' groups said.
The National Weather Service alerted growers to the danger so they could take precautions, but there may still be a heavy loss, meteorologist Jeff Barlow said.
"They won't be able to save all of the crops," Barlow said. "This is going to be a pretty significant freeze event for the central California citrus crop."
Nevertheless, California Citrus Mutual spokeswoman Alyssa Houtby said the group's members were optimistic that damage would be curtailed enough to avoid major shortages or price increases. Because citrus fruit is harvested in winter, farmers are used to dealing with frost, she said.
She said about 25 percent of the citrus harvest had already been picked, and the rest was still on the trees.
'IT GETS A LITTLE DICEY'
Cold weather in moderation can increase the fruit's sugar content, making it more resilient to future touches of frost, Houtby said.
"It can be a good thing for citrus, but when it gets down into the lower 20s for long times, as it is expected to tonight, it gets a little dicey," she told Reuters.
The lemon crop, primarily grown farther to the south in Ventura County, also faced the prospect of frost over the weekend, Houtby said.
California's strawberries, valued at roughly $2.2 billion a year, are especially vulnerable to the cold snaps, with plants generally unable to withstand temperatures lower than 34 degrees for extended periods of time, said Carolyn O'Donnell of the California Strawberry Commission.
Although much of the state's 855,000-ton annual strawberry yield comes from Watsonville, just south of San Jose, the harvesting season there is still months away. But the current U.S. fresh strawberry supply depends on farmers in Ventura County, who produce far less.
Because of the already low production, the effect of frost damage to Ventura County crops at this time of year could be amplified, O'Donnell said.
However, like citrus growers, strawberry farmers are taking precautions, such as running large fans and heaters in the fields.
Among other quintessential Californian crops threatened are avocados, a $400 million industry generally harvested between March and October. The largest growing areas are in San Diego and Ventura counties, said Jonathan Dixon, research program director at the California Avocado Commission.
"The growers I know are having a lot of sleepless nights at the moment," Dixon said. "Most of these guys will be up all night chasing their frost protection."
Although no further snow was expected over the weekend, temperatures were expected to continue to drop on Saturday before gradually warming next week, the weather service said.
In normally temperate San Diego, temperatures were expected to reach 39 degrees on Friday night, closer to the record low of 34 degrees set in 1888 than the normal 59 degrees, said Robert Balfour, a National Weather Service forecaster.
"The rest of the country is probably laughing at us, saying, 'You call that cold?'" Balfour said.